This invention relates to nonmagnetic, dielectric compositions of matter which have large specific heats at low temperatures and their use in low-temperature, cryogenic applications.
The development and use of low temperature processes has greatly expanded in recent years. The space program has spurred action in liquefaction of many different gases including nitrogen, oxygen, helium, and hydrogen. Additionally, the liquefaction of natural gas for large-scale ship transport has been greatly increased as demands for energy in this country have grown.
In many cryogenic applications, the materials used must have large specific heats at the low operating temperatures encountered. For example, the solid packing material used as a heat exchange medium in the regenerator section of closed-cycle stirling-type refrigerators must not only be mechanically stable, but also must have a high specific heat at low temperatures to match closely the specific heat of the refrigerant being utilized for maximum operating efficiency. This is particularly true when helium gas is the refrigerant because at temperatures below 20.degree. K., its specific heat becomes very large. A specific heat mismatch between the solid packing material and refrigerant results in a loss of efficiency.
Other cryogenic applications also require materials with a large low-temperature specific heat. The specific heats of all of the materials used as superconducting wires are quite small at low temperatures. Therefore, the application of a coating of a material with a large specific heat at low temperatures will result in improved thermal stability of the superconductor. Still other cryogenic applications may require materials with special combinations of properties. These properties include a large thermal conductivity at low temperatures, mechanical stability, resistance to cyclic fatigue or cryogenic embrittlement, a nonmagnetic nature, and a nonconductor of electricity.
A large number of prior art materials have one or more of the above properties. These include lead (Pb) which is nonmagnetic and has a large low-temperature specific heat and neodymium (Nd), europium selenide (EuSe), and alloys of erbium, gadolinium, and rhodium (Er-Gd-Rh). However, all of these materials are electrical conductors; in fact, lead is a superconductor at low temperatures.
Even though lead is the most widely used material, it suffers from several shortcomings. It is a relatively soft material with poor creep and impact fatigue properties. In use in the regenerator section of cryogenic cooling systems it tends to degrade into a powder because of cyclic fatigue, and cryogenic embrittlement. Even when hardened by the addition of small amounts (up to 4%) of antimony and made into small spheres, longitudinal thermal conductance between spheres and the breakdown of the spheres into powder shortens the useful life of lead as a heat exchange material in a cryogenic regenerator.
Thus, although some of the materials used by the prior art have one or more of the desirable properties, to my knowledge prior to my invention there were no nonmagnetic dielectric insulating materials having large low-temperature specific heats in use in the art. Accordingly, the need exists in the art for an improved material for use in cryogenic applications which has a large low-temperature specific heat as well as mechanical stability. Additionally, there is a need for a material which combines the above properties with those of being nonmagnetic and a nonconductor of electricity which can be adapted to a wider range of utilities at cryogenic temperatures.